Radial spread of aequorin Ca2+ signal in single frog skeletal muscle fibers.

The Ca(2+)-sensitive photoprotein aequorin was injected into single frog skeletal muscle fibers, and the intracellular aequorin light intensity during muscle activation with different maneuvers was mapped with digital imaging microscopy. During 50 Hz electrical activation (tetanus), the aequorin light intensity from different locations in the muscle fiber rose with very similar time course. Caffeine (10 mM) application, on the other hand, caused aequorin light signals to show significantly different time courses, with an earlier increase in Ca2+ concentration near the surface of the fiber than near the core. The non-uniform rise of intracellular Ca2+ concentration with caffeine treatment is consistent with the slow inward diffusion of caffeine and subsequent Ca2+ release from sarcoplasmic reticulum.